deps: :jpm/module coordinates; the janet.* bridge autoloads jpm modules

The vendored spork/http is gone — jpm owns janet packages. In its place:

- The janet.* bridge autoloads jpm-installed modules on first reference:
  janet.spork.http/server requires spork/http from the module path and
  caches its bindings (failures are negatively cached). Works for any
  module, in every mode, including inside net/server connection fibers.

- deps.edn grows a :jpm/module coordinate: jolt-deps verifies the module is
  importable at resolve time, optionally running `jpm install` on the
  :jpm/install package once when it isn't, and otherwise fails with the
  install hint. Contributes no source roots. ring-app declares spork/http
  this way.

Docs: README's interop section, docs/tools-deps.md (:jpm/module reference),
and the ring-app README (including the jpm-version caveat for spork HEAD's
.janet native sources, which older jpm rejects).

docs/tools-deps.md

@@ -107,3 +107,23 @@ a regex feature like Unicode property classes (`\p{…}`).
 - **Compiling deps into a binary image.** `uberscript` already produces a
   standalone `.clj`; baking a project's dependencies directly into a custom
   executable image is a heavier variant that isn't implemented.
+
+## Janet dependencies: `:jpm/module`
+
+A jolt project can depend on janet libraries. jpm owns their installation;
+`deps.edn` declares the requirement and `jolt-deps` verifies it at resolve
+time:
+
+```clojure
+:deps {janet/spork-http {:jpm/module "spork/http"
+                         :jpm/install "spork"}}
+```
+
+- `:jpm/module` — the janet module path that must be importable.
+- `:jpm/install` (optional) — the jpm package to install when it isn't;
+  `jolt-deps` runs `jpm install <name>` once, then re-checks. Without it the
+  resolve fails with the install hint.
+
+A `:jpm/module` dep contributes no source roots. At runtime the `janet.*`
+interop bridge autoloads the module on first reference
+(`janet.spork.http/server`, …), so nothing else is needed.